The invention is particularly concerned with weighing apparatus comprising a weigh cell of the spring balance type which produces an electrical output signal proportional to weight. A number of conditions encountered during operation of such a weigh cell serve to degrade the accuracy of the weigh cell. For example, during the net or gross weight filling of commodities in the form of large pieces, the impact of the pieces, upon landing, causes undesired signals which mask or otherwise interfere with the weight signal. Similarly, vibrations transmitted to the weigh cell and electrical pick-up, e.g., switching transients, also contribute unwanted signal components to the weight signals. These unwanted signals or signal components, hereinafter referred to as noise, are superimposed upon the weight signal and thus place a limit on the smallest increment of weight that can be accurately detected by the weigh cell.
The effect of the impact of individual pieces of the commodity to be weighed upon the weighing accuracy can be readily appreciated by considering the fact that a potato chip landing edgewise upon a weigh hopper can produce a momentary signal twice as large as the same chip when landing flat upon the other chips in the weigh bucket. A large chip will weigh about 1/8 oz. so that the impact caused thereby could result in a weighment which actually is 1/8 oz. lighter than the weight cut-off point, because the chip in landing on its edge would be perceived by the scale as weighing 1/4 oz.
The vibration problem referred to above dictates the use of slower weigh cells. To explain, a given spring suspension has the shortest settling time when the suspension is critically damped. Under these conditions the settling time is equal to one cycle of the natural frequency of the suspension. If the suspension is stiffened so as to double the natural frequency, the travel of the suspension for a given weight increment will be reduced by a factor of four. In other words, a given vibration amplitude produces a signal representing four times as much weight when the speed of response of the suspension is doubled.
Considering a specific example dealing with vibration, a 1/10 second weigh cell with a weigh hopper, moving members and a load of 62 oz., will only move 25 millionths of an inch (0.000025") when 1/64 oz. is added. However, the vibration for a conventional 1/10 second weigh cell of an automatic weighing machine will substantially exceed that corresponding to the signal associated with a weight of 1/64 oz. and thus the limitations vibrations place on the accuracy of the weigh cell are evident.
It is also noted that the vibration frequencies encountered under operating conditions vary from about 0.3 to 60 hertz (Hz), while the natural frequencies of weigh cells vary between 3 to 10 Hz. A multiple scale weighing machine can include twenty feeders which turn on and off at various times. The drive motors for the machine can produce frequencies from 20 to 30 Hz while the associated V-belts, gears and oscillating members generate a multitude of frequencies. Further, sealing jaw carriages, crank arms and former carriers generate frequencies below the natural frequencies of the weigh cells. In addition, some of the most severe vibrations are transmitted through the floor, these vibrations resulting from passing lift trucks, conveyors, vibratory product distribution systems and the like. Thus, the vibrations referred to above appear in the weight signal produced by the weigh cell as a mixture of components of varying frequencies and changing phase relationships.
The electrical pick-up noise mentioned above can be either radiated or conducted noise and the most serious is caused by voltage transients. Vibratory feeders can generate 2,000 volt transients while solenoids and relays also generate substantial transients.
As explained in more detail hereinbelow, the present invention affords substantial improvement in the ratio of the weight signal produced by a weigh cell relative to noise caused by impact, vibration and electrical pick-up, and one aspect of the invention concerns the provision of mechanical damping for this purpose. Two patents which are relevant to this aspect of the invention are U.S. Pat. No. 2,793,026 (Giardino et al.) and U.S. Pat. No. Re. 28,303 (Blodgett).
The Giardino et al. patent discloses a spring balance weighing apparatus for rapid weighing and checking operations including a pair of spring suspensions which are individually connected to a common support through respective damping mechanisms. One of the suspensions includes a scale pan and a capacitor plate is associated with each suspension. Relative movement between the plates (as occurs when the suspensions move relative to one another) causes a change in the spacing between the capacitor plates and thus a change in the output signal sensed by an associated electrical measuring instrument. The Giardino et al. patent provides an adjustment for preventing unwanted intercoupling of the two suspensions.
The Blodgett patent discloses dynamically compensated weighing scales wherein a velocity sensitive coupling is provided between a scale pan suspension and a compensator suspension. The patent also states that damping of the scale pan suspension may be desirable. The emphasis in the patent is on impact compensation and rate sensing. The subject matter of the Blodgett patent is hereby incorporated by reference.